This invention relates to a motor vehicle hydraulic fluid power circuit having a hydraulic pump for supplying fluid pressure to assist the operator in steering and braking the vehicle.
Motor vehicles are frequently provided with a hydraulic pump which is driven by the engine of the vehicle to provide a source of fluid power. In passenger cars, this source of fluid power may be used in a fluid power circuit to assist the operator in steering the vehicle and to assist the operator in braking the vehicle.
In such motor vehicle fluid power circuits, it is also desirable to provide one ratio of brake pedal movement to master cylinder movement under normal operating conditions and to provide a different ratio in the event of a fluid pressure failure in the system. This is desirable because one such ratio will provide the brake pedal movement and brake pedal feel characteristics to which drivers are now accustomed with vacuum actuated braking systems, while a different ratio is required to provide high braking pressures with low pedal effort in the event of pump failure.
In order to provide one ratio of brake pedal movement to master cylinder movement during normal operation and a second ratio in the event of pump failure, both mechanical and hydraulic ratio change devices have been proposed. Mechanical ratio change brake boosters are shown in U.S. Pat. Nos. 3,733,966 and 3,733,968. Hydraulic ratio change brake boosters are shown in U.S. Pat. No. 3,793,829 (which provides a ratio change at the outlet end of the brake booster) and in U.S. Pat. Nos. 3,831,491 and 3,838,629 (which provide a ratio change at the inlet end of the brake booster). Another hydraulic ratio change brake booster is shown in applicant's copending U.S. patent application Ser. No. 594,804 filed July 10, 1975, and assigned to the assignee of this application. An annular sleeve for changing the ratio between inlet and outlet pressure of a brake line pressure valve is shown in U.S. Pat. No. 3,832,007.